The problem with Dead Space is that the ammo drops are based on your current guns. As you buy more, the enemies drop all types of ammo for each. Except when you REALLY want a certain type of ammo.
So the solution is to stick to one or two guns and maximize the drops. This also lets you upgrade guns much easier.
Plasma Cutter + Line Gun is a good choice.
Since you don't care about achievements (Scenester's mention of Plasma Cutter requires you to beat the game using only it), just pick what guns you want to use and have fun.